U.S. Defense Secretary Lloyd Austin said on Thursday the process of removing all contractors from Afghanistan working with the United States was under way as part of President Joe Biden's withdrawal of forces from the country.

The remarks are the clearest indication yet that Biden's April order to withdraw all U.S. forces from Afghanistan by Sept. 11 extended to U.S.-funded contractors.

Asked whether the Pentagon had issued orders to withdraw not just American troops but also contractors, Austin said: "We're going to responsibly retrograde all of our capabilities that we are responsible for and the contractors fall in that realm as well."

Speaking with reporters, Austin said the contractors could, however, renegotiate their contracts in the future.

As of April, there were nearly 17,000 Pentagon contractors, including about 6,150 Americans, 4,300 Afghans and 6,400 from other countries.

The departure of thousands of contractors, especially those serving the Afghan security forces, has raised concerns among some U.S. officials about the ability of the Afghan government and military to sustain critical functions.

Austin said the drawdown was going according to plan so far.

But Afghan security forces are locked in daily combat with the Taliban, which has waged war to overthrow the foreign-backed government since it was ousted from power in Kabul in 2001.

In just two days, the Taliban captured a second district in the northern province of Baghlan on Thursday.